Road toll systems for charging tolls for vehicles by means of roadside radio beacons that communicate with onboard units (OBUs) on the vehicle side in order to identify and locate these are known. However, it is often necessary to detect the presence of a trailer, since this determines the extent of the toll. Conventionally, to detect the presence of a trailer, either the number of axles is determined in order to deduce the presence of a road train with trailers from a specific number of axles, or the presence of a trailer must be manually input by the user at the OBU of the vehicle. The latter requires complex systems for counting axles such as laser scanners or induction loops in the road surface, the latter being prone to error and manipulation.